1) Field of the Invention
The invention relates to storage-stable, silane-modified core-shell copolymers, to processes for preparing storage-stable, silane-modified core-shell copolymer dispersions and powders, and to their use.
2) Background Art
Alkoxysilane-functional copolymers are frequently employed in practice as building adhesives (DE-B 2148456=GB-A 1407827) or for preparing emulsion paints and polymer-based plasters (DE-A 2148457=GB-A 1407827, EP-A 327006=U.S. Pat. No. 5,576,384). The alkoxysilane groups are incorporated by polymerization into the copolymers in order to improve the wet adhesion and the water resistance in the case of use as coating materials, and to improve the adhesion to mineral substrates in the case of use as building adhesives or tile adhesives.
A problem when using alkoxysilane-functional copolymers, especially in the form of their aqueous dispersions, is their tendency to premature crosslinking through hydrolytic condensation reactions of the alkoxysilane groups. As a consequence, aqueous dispersions of alkoxysilane-functional copolymers frequently possess inadequate stability on storage and when stored for several months lose their very good binder properties.
EP-B 687277 discloses aqueous dispersions of core-shell copolymers which are obtainable by a procedure in which the alkoxysilane-functional comonomers are reacted with a small amount of predominantly hydrophilic comonomers to give a water-swellable or water-soluble addition polymer which subsequently, in the polymerization of the major amount of hydrophobic comonomer, envelops the hydrophobic core formed in the polymerization. This gives core-shell polymers in which the alkoxysilane-functional comonomer units are present externally in the shell, with the effect that very good adhesion properties result with relatively small amounts of alkoxysilane-functional comonomer. A disadvantage, however, is that after these dispersions have been stored for six months the very good binder properties (adhesive strengths under tension following dry and wet storage) in dispersion tile adhesives and plasters are no longer achieved.
EP-A 444827 discloses silane-modified core-shell copolymer dispersions, for preparing elastic coatings, which comprise a core of vinyl ester-olefin-acrylate-vinylsilane copolymer and a vinyl ester-olefin shell. For their preparation, part of a mixture of vinyl ester, a acrylate and vinylsilane is introduced as initial charge, ethylene is injected, and the remainder of the comonomer mixture is metered in. After the end of the metered addition of silane-containing monomer, a second metered, silane-free vinyl ester addition is started. This process achieves the formation of a silane-containing, hydrophobic core, which has particular strength owing to crosslinking by way of the silane functions, and the formation of a silane-free shell. Although the silane component of these core-shell polymers is concentrated in the core and surrounded by a shell, the tensile adhesion values achieved following wet storage with this system when used as a tile adhesive are extremely poor.
From EP-A 366969 (U.S. Pat. No. 5,100,948) it is known that ceramic tile adhesives based on aqueous dispersions of polymers comprising mercaptosilane rather than vinylsilane units exhibit greater water resistance in the solidified state than do vinylsilane-containing polymer dispersions.
The object, therefore, starting from the silane-modified core-shell copolymers of EP-A 687277, was to provide storage-stable, silane-modified core-shell copolymer dispersions with which high-grade binder properties are obtained via the crosslinking of the silane functions even after storage at room temperature for 12 months.